Computer networks at South Korean TV broadcasters and two majors banks are shut down by malicious cyberattacks today amid speculation that the attack originated from North Korea.

Systems at the South Korean military were not affected, but the army raised its defense level.

North Koreans last week had blamed South Korea and the United States for temporarily shutting down their websites in Pyongyang. For weeks, the communist state has been ratcheting up threats against South Korea and the U.S..

Officials at KBS, MBC, and YTN broadcasters said their computers were shut down at 2 p.m. and have not been able to reboot. LGU+, the network provider to these companies, said they see three skull drawings on a black screen with the phrase, "hacked by Whois Team," which experts say is a secretive group of hackers. The broadcasters are currently capable of airing programs as normal, though.

Shinhan Bank and NongHyup Bank computers were also temporarily shut down. Shinhan Bank, a lender of South Korea's fourth-largest banking group, took a hit on its online banking and automated teller machines, but its servers were back up within two hours.

Police officials are declining to speculate on whether North Korea is behind this attack, but says the incident is "pretty massive" and it will take a few days to collect conclusive evidence.

Others, however, are saying the likely culprit is North Korea.

"Lately they [North Korean leaders] have been stating publicly that they will make a revenge attack," Lim Jong-In, professor of information security at Korea University, told ABC News. "This sort of mass scale attack is a planned organizational one, not by some hacker."

"North Korea wants to show-off their strong arm without making human casualties. Their goal is to create instability here," Lim said.

In addition, previous statements from North Korea's NKTV specifically named KBS and MBC last April saying these TV stations "will come under fire in an unimaginable and unusual way."

North Korea's official Central News Agency had accused South Korea and the U.S. last week of an aggressive stance against the regime with "intensive and persistent virus attacks."

Experts indicated it could take months to determine what happened and one analyst suggested hackers in China were a more likely culprit.

North Korea has staged several cyber attacks targeting South Korea and U.S. institutions since 2009. The two major ones were in 2011 which knocked out South Korean servers at banks, conservative newspapers, and institutions in the form of a denial of service attack which experts believe was an effort to test South's computer defenses.

Wednesday's attack was in the form of hacking and planting malicious software or a virus.

"Their cyber attack method is advancing and unfortunately, there can not be a perfect defense in the future," said Joseph Yoon, IT specialist at Korea Telecom. "These attacks will not result in casualties, but such indirect attack on social infrastructure creates instability and distrust among the South Korean citizens towards the government."

Lim said North Korea is an expert at hacking.

"North Korean hacking skills are one of the world's top five. It's not a matter of how weak South Korean is in defending the attacks. We are talking about determined attackers who work non-stop to hack their hostile enemy, South Korea and the U.S.," warned Lim.

ABC News' Cho Long Park, Joanne Kim, and The Associated Press contributed to this story.